A hydrate is a solid substance, water ice, composed of water molecules and gas molecules. Hydrates are formed by brining gas molecules into contact with water at a predetermined pressure and temperature, and the hydrates can be dissociated back into water and gas molecules by changing the pressure or temperature.
Hydrates have attracted attention as an alternative means for transporting and storing natural gas, which can substitute for LNG, due to its high gas-retaining property.
FIG. 1 is a diagram illustrating the construction of a conventional gasifying apparatus for gasifying hydrates of natural gas back into gas.
For example, Korean Patent Application No. 10-2009-0077592 discloses a gasifying apparatus for gasifying hydrates of natural gas. According to Korean Patent Application No. 10-2009-0077592, as illustrated in FIG. 1, the gasifying apparatus includes at least one inlet 102, through which hydrates are continuously introduced, a guide member 104, which guides hydrates so that the hydrates are brought into contact with a heating means to be gasified, a gas outlet 106, which is disposed at an upper end of the apparatus and through which gasified natural gas is discharged outside, and a drain which is installed at the bottom and through generated water is drained.
Generally, Natural Gas Hydrates (NGHs) are stored and transported in the form of NGH pellets in a large volume tank. In this case, there is a problem that hydrate pellets adhere to each other due to their own weights.
For gasification or dissociation (hereinafter, collectively referred to as dissociation) of hydrate pellets, there are two conventional dissociation methods: a method of fracturing the adhered hydrate pellets on a ship, transporting the fractured hydrate pellets to land, and dissociating the fractured hydrate pellets on land; and a method of directly dissociating hydrate pellets by heating the tank by means of hot water or heating wire on a ship.
Both of these methods have a problem that the ship, on which a hydrate storage tank is mounted, needs to be docked in a port for a long period of time either while the hydrate pellets, which are adhered to each other in a tank during storage and transportation of the hydrate pellets, are being fractured or pulverized and are then being moved to facilities for dissociation on land; or while the hydrate pellets are being directly dissociated by heat on the ship.
Long lay-over time which is required for loading/unloading and dissociation of hydrates is the main factor which decreases ship operation efficiency. Furthermore, a ship which sails back to a site where hydrate pellets are loaded on a ship, usually sails back with an empty cargo tank.
Generally, when hydrate pellets are just loaded in a cargo tank on a ship, the hydrate pellets are separated from each other so that grain stability is used to assess the ship stability. However, since hydrate pellets come to adhere to each other over time due to their own weights, this adhesion negatively influences ship's behavior and stability, depending upon the adhesion state within the tank.
In addition, when a ship uses a Dual Fuel Engine (DFE) to use Boil Off Gas (BOG) generated from hydrate pellets as a fuel when transporting hydrate pellets stored in a large volume tank, the characteristics of the fuel vary, because propane happens to evaporate earlier than methane in some cases, depending on the compositions of the hydrate pellets. Furthermore, when unloading the hydrate pellets from a ship, the energy and composition of dissociated gas are likely to be non-uniform.
In addition, when melting and dissociating hydrate pellets stored in a large volume tank, hot water may be used. In this case, surfactants are usually added to the hot water to prevent formation of ice within the tank. Since the surfactant-containing hot water causes environmental pollution, additional facilities to treat wastewater are needed in the process of dissociation of hydrates, resulting in an increase in cost.